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It seems like a conspiracy to keep everyone from knowing this.
Custom cape sellers don´t say what it is, the women from the draperies I questioned always say something along the line of "coated propylene" which is no longer in the making because the only company who sold it went bankrupt in 2003.
Even LEGOs customer service can only say "Acrylic matte medium" which apparently is some kind of paint artists use to paint on canvas.
The term acrylic is used for many mediums, like acrylic paint (liquid), acrylic fiber (clothing), acrylic rubber (flexible) and acrylic glass (really hard).
So, as you see, it is possible to make those capes out of a type of acrylic material.
It is fabric - probably cotton or polyester - coated with an acrylic substance that makes it more durable and tear-resistant. You can easily print on acrylic-coated fabric using standard laser printers.
You can purchase acrylic-coated fabric through suppliers that sell sign-making supplies as well as art supply shops.
see this tutorial on Creating custom fabric elements for Lego minifigures http://www.minifigcustomizationnetwork.com/howto/899
I've used iron-on patch material (the thinner twill variety that comes in bright colors) with success for my kids' minifigs. I cut it out with fabric scissors and use a good hole punch for the holes. Takes about one to three minutes a cape depending on complexity and is $2.47 at Walmart for a 4 pk of 5"x5" patches.
Paint everyday fabric with black acrylic paint. Use a paint brand like AppleBarrel. This will not give you fabric exactly like LEGO, but it is durable, cutable, and can be hole punched. You can paint over it with any color once the initial paint has dried.
NOTE: This was not my idea. It came from a YouTube video by NobleArtist. Check out his channel.
